What type of immune cell is primarily responsible for attacking virally infected cells lacking class I MHC?

Natural killer (NK) cells are primarily responsible for identifying and targeting virally infected cells that lack class I major histocompatibility complex (MHC) molecules. In a healthy state, most nucleated cells in the body express class I MHC molecules, which present endogenous antigens (like those from viruses) to CD8 T lymphocytes, signaling that the cell is healthy. However, many virally infected cells downregulate or lose these class I MHC molecules to evade detection by the adaptive immune system.

NK cells play a crucial role in the innate immune response, particularly in recognizing and killing these MHC-deficient cells. They have activating and inhibitory receptors that allow them to sense when a target cell is abnormal, often due to viral infection. When an NK cell detects low or absent MHC class I expression, it can initiate cell death pathways, effectively eliminating the infected cells and curtailing the spread of the virus. This rapid response is vital, especially in the early stages of viral infections before the adaptive immune system is fully activated.

In contrast, CD8 T lymphocytes specifically require class I MHC molecules to recognize and respond to infected cells. B lymphocytes are mainly involved in antibody production and humoral immunity, while